Scanning electrochemical microscopy

ABSTRACT

A new scanning electrochemical microscopy tip positioning method that allows topography and surface activity to be resolved independently is presented. A SECM tip is oscillated relative to the surface of interest. Changes in the oscillation amplitude, caused by the intermittent contact of the SECM tip with the surface of interest, are used to detect the surface of interest, and as a feedback signal for various types of imaging.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national phase filing, under 35 U.S.C.§371(c), of International Application No. PCT/GB2011/050747, filed Apr.14, 2011, the disclosure of which is incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

This invention relates to scanning microscopy.

BACKGROUND TO THE INVENTION

Scanning electrochemical microscopy (SECM) is a scanned probe microscopytechnique in which the electrochemical response of a SECM tip (typicallyan ultramicroelectrode (UME) with an active part of less than 25 μm) isused to provide information on the properties (e.g. topography orchemical activity) of a surface of interest (an interface, surface orphase) or for modification of a surface of interest (an interface,surface or phase). The SECM tip is immersed in a solution and used todetect chemical species (molecules or ions) which interact with the SECMtip or to generate chemical species (molecules or ions). Because theSECM tip detects or generates concentrations and fluxes of a chemicalspecies locally, it provides information on the properties of aninterface, surface or phase at high spatial resolution. The SECM tip canbe operated in amperometric, potentiometric or conductivity modes,amongst other possibilities. A wide variety of SECM tips have beendescribed including solid metal electrodes, semiconducting electrodesand liquid/liquid probes.

Conventional amperometric SECM (which forms the majority ofapplications) typically operates in direct current (DC)-constant height(CH) mode in which the tip, usually held at a potential to detect orelectrolyze an analyte at a diffusion-limited rate, is positioned abovethe interface of interest. The tip response, recorded as a function ofthe tip position, provides a current image which depends on both thesample topography (distance between the tip and the interface) andsurface activity.

Despite the tremendous impact of SECM in interfacial science, asignificant—and generally unresolved—challenge concerns absolute tippositioning. There is a need for methods which allow the topography(distance between the tip and the interface) and surface flux (oractivity) information to be determined simultaneously and unambiguously.

Several methods have been proposed in the prior art to address thisgeneral issue, including:

-   -   the use of two electroactive mediators in the solution; one        which maps the topography alone and the other the activity; (see        “Scanning electrochemical microscopy as a local probe of oxygen        permeability in cartilage.” Gonsalves, Marylou, Anna L. Barker,        Julie V. Macpherson, Patrick R. Unwin, Danny O'Hare, and C.        Peter Winlove, Biophysical Journal, 2000, 78(3), pp 1578-1588    -   the use of impedance based methods for tip positioning and        feedback; (see “Impedance Feedback Control for Scanning        Electrochemical Microscopy” Mario A. Alpuche-Aviles and David O.        Wipf, Anal. Chem., 2001, 73 (20). pp 4873-4881)    -   the use of tip position modulation (TPM) to detect the substrate        surface and control the positioning of the SECM tip; (see U.S.        Pat. No. 5,382,336) the use of shear force methods for tip        positioning and feedback; (see “Topography feedback mechanism        for the scanning electrochemical microscope based on        hydrodynamic forces between tip and sample”, Markus Ludwig,        Christine Kranz, Wolfgang Schuhmann, and Hermann E. Gaub, Review        of Scientific Instruments, 1995, 66, pp 2857-2860 and “Combined        scanning electrochemical/optical microscopy with shear force and        current feedback”, Youngmi Lee, Zhifeng Ding, and Allen J. Bard,        Anal. Chem., 2002, 74, pp 3634-3643, and    -   the integration of a UME into atomic force microscopy tips        (SECM-AFM). (see “Combined Scanning Electrochemical-Atomic Force        Microscopy J. V. Macpherson and P. R. Unwin, Anal. Chem., 2000,        72, 276-285, available at the time of writing at        http://dx.doi.org/10.1021/ac990921w)

However, it can be difficult to find appropriate redox-active speciesthat do not interact with the underlying system of interest when usingtwo mediators.

Also, the impedance feedback and TPM methods depend on the measuredcurrent to determine the distance from the SECM tip to the substratesurface and thus are dependent on the activity of underlying substrate.

The shear force feedback and SECM-AFM methods are attractive as theyutilize a force feedback signal to allow a close (and more or lessconstant) separation to be maintained between the tip and interface ofinterest during imaging, but they require additional specialistinstrumentation and/or non-conventional tips.

SUMMARY OF THE INVENTION

A first aspect of the invention provides a method comprising:

-   -   oscillating a scanning microscopy probe tip relative to a        surface of interest;    -   detecting damping of an amplitude of the oscillation of the        probe tip resulting from the probe tip coming into contact with        the surface of interest;    -   using the detected damping to detect the surface of interest;        and    -   using the probe tip to measure or modify activity of the surface        of interest simultaneously with detecting damping.

The invention also provides a computer program comprising machinereadable instructions that when executed by scanning electrochemicalmicroscopy apparatus control it to perform the method

A second aspect of the invention provides a computer readable mediumhaving stored thereon machine readable code that when executed by aprocessor of a scanning microscopy apparatus cause the apparatus toperform a method comprising:

-   -   oscillating an probe tip relative to a surface of interest;    -   detecting damping of an amplitude of the oscillation of the        probe tip resulting from the probe tip coming into contact with        the surface of interest;    -   using the detected damping to detect the surface of interest;        and    -   using the probe tip to measure or modify activity of the surface        of interest simultaneously with detecting damping.

A third aspect of the invention provides apparatus configured:

-   -   to oscillate a scanning microscopy probe tip relative to a        surface of interest;    -   to detect damping of an amplitude of the oscillation of the        probe tip resulting from the probe tip coming into contact with        the surface of interest;    -   to use the detected damping to detect the surface of interest;        and    -   to use the probe tip to measure or modify activity of the        surface of interest simultaneously with detecting damping.

A fourth aspect of the invention provides apparatus comprising:

-   -   means for oscillating a scanning microscopy probe tip relative        to a surface of interest;    -   means for detecting damping of an amplitude of the oscillation        of the probe tip resulting from the probe tip coming into        contact with the surface of interest;    -   means for using the detected damping to detect the surface of        interest; and    -   means for using the probe tip to measure or modify activity of        the surface of interest simultaneously with detecting damping.

A general discussion of salient features of the embodiments andassociated effects and advantages now follows.

The invention, some aspects of which are known as intermittent contactSECM (IC-SECM), is a new scanning microscopy, for instance SECM, tippositioning method that can allow topography and surface activity to beresolved independently.

Embodiments of the invention involve the application of an oscillatoryperturbation to the SECM tip relative to the surface of interest. Theheight of the SECM tip oscillates about the average SECM tip height. TheSECM tip is moved relative to the surface of interest and the amplitudeof the oscillation becomes damped as the SECM tip encounters thesurface. The damping is detected and provides absolute information onthe SECM tip-surface separation. The damping of the oscillationamplitude is used as a feedback signal to control the movement of theSECM tip relative to the substrate surface. The electrochemical responseof the SECM tip can be either measured to provide complementaryinformation about the interface, surface or phase or controlled toprovide modification of the interface. surface or phase. Aspects of theinvention can be employed with all SECM tip types (includingamperometric and potentiometric) and in all SECM configurations. Aspectsof the invention can also be employed with nanoscale SECM electrode tipsand SECM tips which have been shaped by focussed ion beam methods orpolishing and etching procedures.

Aspects of the invention can also be used for positioning and imagingwith ion conductance probes and fibre optic probes, and other probesthat it may be desired to position physically near a surface orinterface, such as probes for local gas detection/imaging; positioningmicrofluidic cells etc. The SECM tip may be a potentiometric indicatorprobe.

Compared to existing techniques, embodiments of this invention providean inexpensive, robust and simple method to independently resolvesubstrate topography and surface activity. Minimal additional equipmentand control schemes are needed to implement certain embodiments of theinvention. Thus, aspects of the invention provide an inexpensive andsimple method to resolve substrate topography and substrate activity inSECM. The feedback (used to locate the surface) depends on the physicalinteraction of the SECM tip with the substrate surface and does notdepend on the surface activity. This means that changes in substrateactivity do not affect the tip-substrate distance measurements, and thusembodiments of the invention provide a robust method to determinesubstrate topography and substrate activity independently. This alsomeans that aspects of the invention can be used with standard SECM tips(UMEs) and in standard SECM modes. Aspects of the invention can be usedalso in non-electrochemical microscopy.

A particularly attractive feature of this approach is that it wouldallow other types of SECM tips, such as potentiometric electrodes andion conductance probes, to be deployed. Also because the tip position isoscillated slightly, one can additionally isolate the ac component ofthe current as well as the mean current, greatly enhancing theelectrochemical information content.

Embodiments of this invention differ from Tapping Mode AFM (TM-AFM)(U.S. Pat. No. 5,412,890) in that the embodiments use a completelydifferent tip design (as compared to the cantilever construction tipsused in TM-AFM), and the tip is oscillated at different frequencies (ascompared to resonance frequencies used in TM-AFM).

This invention differs from the tip position modulation (TPM) method ofWipf and Bard (U.S. Pat. No. 5,382,336) in that a non-electrochemicalsignal is used to to provide the feedback and positioning sensitivity.Unlike TPM, the SECM tip comes into contact with the substrate with thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 shows a UME and a substrate surface as used with embodiments ofthe present invention;

FIGS. 2 and 3 show movement of SECM tips according to embodiments of theinvention; and

FIG. 4 shows an implementation of intermittent contact SECM embodyingaspects of the invention.

DETAILED DESCRIPTION

Embodiments of the invention involve the application of an oscillatoryperturbation of typically 0.1 nm to 1 μm at typically 5 to 100 000 Hz toa SECM tip. The SECM tip may be a UME, for instance in the form of ametal wire A, with a radius of 0.002 to 12.5 μm, sealed in a glasscapillary B relative, typically normal, to a surface of interest C. Withthis oscillation the amplitude of the oscillation becomes damped as thetip encounters the surface. The oscillation is typically sinusoidal,though other oscillation types can be used.

FIG. 1 shows a cross section of an UME and the oscillation of the UME.FIG. 2 shows the movement of the SECM tip when in bulk solution (i.e.not in contact with the surface). FIG. 3 shows the movement of the SECMtip when in intermittent contact with the substrate surface. Here, itcan be seen that the uppermost half of the waveform, i.e. the partsabove position 0, are substantially the same as for FIG. 2, however thelowermost half of the waveform is damped by contact with the surface C.The damping is detected and provides absolute information on thetip-surface separation. This can be used as a measure of tip-surfaceseparation for approach curve measurements (where the tip is translatedtowards or away from the substrate, usually in a perpendiculardirection) and as a set point to maintain a fixed distance between thetip and substrate surface during imaging (where the tip and/or thesubstrate are moved laterally with respect to each other). The SECM tipelectrochemical signal (e.g. current and/or potential) is measuredthroughout and provides information about the surface activity.

A detailed description of an implementation of the invention is nowgiven. Embodiments of the invention are described with amperometricapproach curve measurements and amperometric and potentiometric imaging.The amperometric configuration for an electrically unbiased sample isshown schematically in FIG. 4. Note that if the sample is a conductor ofsemiconducting material, it can also be connected up as an electrode(potential and/or current control).

Apparatus and Instrumentation

Coarse control of the SECM tip B (a Pt disk UME), which is typicallymounted perpendicular to the substrate surface 52 but can be mounted atdifferent angles, is realised by a three-dimensional manual x,y,z stagecontrolled by manipulator screws 56. Note that other means of achievingthis positioning are also possible. Fine control is realised by three(x, y, z) piezoelectric positioners 42 fitted with strain gauge sensors.The piezoelectric positioners 42, operated in closed loop, arecontrolled by an amplifier/servo 47. The piezoelectric positioneramplifier/servo is controlled by a personal computer 45. An ac signalprovided by an AC generator 49 is added to the z piezoelectricpositioner control by a signal adder 48. The ac signal creates asinusoidal oscillation of:

δ*sin(2*πf*t)

in the height of the SECM tip B about the average tip height, but otheroscillation profiles can be used.

The computer 45 includes processing means, comprising one or moreprocessors, memory means, comprising one or more memories, and acomputer program stored in the memory means. The processing means, undercontrol of the computer program, performs various actions that aredescribed below, including measuring, detecting and controllingoperations.

Embodiments of the invention are typically implemented in a Faraday cage41 on a vibration isolation table 55 in a two electrode arrangement withthe metal wire A or UME tip B as the working or active electrode and aquasi-reference electrode 51. However, a three electrode (working,reference and counter electrodes) or a four electrode bipotentiostaticsetup can be used, among other well known electrochemical setups.

The SECM tip current and the location of the piezoelectric positionersare recorded. The SECM is operated in a diffusion-limited configuration;with the SECM tip held at a potential to electrolyse a target chemical.

Intermittent contact SECM (IC-SECM) Approach Curves

The SECM tip is moved close to the substrate surface using themanipulator screws 56. Approach curve measurements are carried out bytranslating the SECM tip towards the substrate using the z one of thex,y,z piezoelectric positioners 42. Simultaneously the SECM tip istypically oscillated at a frequency of 70 Hz with a magnitude of 1-2%(10 nm-150 nm) of the active electrode radius. The oscillation magnitudemay, however, take any value between 0.001% and 50% of the activeelectrode radius. The IC-SECM approach curve is terminated whenintermittent contact is detected. Intermittent contact here is definedas a sustained decrease in the z piezoelectric positioner strain gaugesensor (z-SGS) tip oscillation amplitude as compared to the z-SGSoscillation amplitude in the bulk solution (for example a 1 to 15%sustained decrease).

As the UME tip B approaches an insulating substrate surface C the meancurrent decreases. When approached to a conducting substrate C surfacethe mean current increases. The magnitude of the z-SGS oscillationremains constant for most of the approach curve, only changing whenintermittent contact is made between the UME tip B and the substratesurface C.

Although the oscillation frequency here is 70 Hz, it may take any valuebetween 5 and 100 000 Hz. The oscillation frequency may be between 5 and5000 Hz. The oscillation frequency may be between 30 and 110 Hz.

The oscillation amplitude may between 0.1 nm and 1 μm. The oscillationamplitude may be between 5 nm and 500 nm. The oscillation amplitude maybe between 15 nm and 250 nm.

The oscillation amplitude of the SECM tip is monitored, and the measuredoscillation amplitude is used to control the SECM tip movement relativeto the surface of interest.

Instead of a sinusoidal oscillation, it may take some other form. Forinstance, a square oscillation may be applied to the SECM tip. Theoscillation frequency of the square wave may be between 5 and 100 000Hz, or may take some other value. The oscillation amplitude may bebetween 0.1 nm and 1 μm.

A sawtooth oscillation may be applied to the SECM tip. The oscillationfrequency of the sawtooth signal may be between 5 and 100 000 Hz. Here,the oscillation amplitude may be between 0.1 nm and 1 μm.

The electrochemical response of the SECM tip is measured to provideinformation about the surface of interest. The electrochemical responseof the SECM tip may be the current generated at, or flowing through, theSECM tip when held at a potential to interact with a species ofinterest. Alternatively it may be the potential generated at the SECMtip when interacting with a species of interest. It may alternatively bethe potential when a current is applied to the tip, via galvanostaticcontrol. It may alternatively be a conductance current.

The electrochemical response of the SECM tip may be used to deliverchemical species to the surface of interest.

The surface of interest in these embodiments is an interface between twosubstances, a surface of a solid or liquid, or a boundary between twophases (i.e. solid and liquid, liquid and gas, or solid and gas) of asubstance, although it could be another surface such as a surface of aliving cell or tissue.

The SECM tip is oscillated normal, or substantially normal, to thesurface of interest.

Intermittent Contact SECM Imaging

The SECM tip B is engaged to the surface C using an Intermittent Contact(IC)-SECM approach curve which halts when intermittent contact isdetected. An image is constructed typically using a series of linescans, although other scan methods are possible. Each line scan consistsof a forward intermittent contact scan and a reverse constant distancescan. The forward scan is done while maintaining intermittent contactwith the substrate surface C. The reverse scan is done at a constantdistance away from the substrate surface C, which is identified by the zmeasurements of the tip position B in the forward scan. This separationis typically in the range 0.1-2 μm for a 2 μm active radius tip B.During the intermittent contact scan the SECM tip height is updated by aproportional controller, implemented on the computer 45. Other forms ofcontroller, for instance a PID (proportional-integral-derivative)controller, can be used instead.

The proportional controller takes the form:

z _(new) =z _(old) +P*(z ^(SGSAmplitude)−0.9*z ^(SGSBulkAmplitude)),

where z_(new) and z_(old) are the new and old SECM tip heightrespectively, z^(SGSAmplitude) is the z-SGS oscillation amplitude andz^(SGSBulkAmplitude) is the z-SGS oscillation amplitude in the bulksolution.

A ten percent decrease in the z-SGS oscillation amplitude is used as aset point for scanning, although other values can be used. The SECM tipcurrent is measured during the line scans. The images of chemicalactivity (from the various tip current measurements) and substrateheight (from the location of the z piezoelectric positioner) are thusconstructed simultaneously.

On a substrate with conducting and insulating regions, IC-SECM imaging,when the UME is operated in an amperometric feedback mode, produces animage with an increase in mean current over the conducting regions(positive diffusional feedback) and a decrease in mean current over theinsulating regions (negative diffusional feedback). The mean current canbe recorded during both the intermittent contact lines scans and theconstant distance lines scans. The same pattern of increases anddecreases in mean current is observed in both the intermittent contactand constant distance images. However the intermittent contact meancurrent shows a greater variability than the constant distance meancurrent. The substrate surface is identified by the computer 45 by theposition of the z piezoelectric positioner during the intermittentcontact lines scans. In addition, the oscillating component of thecurrent can be isolated. The magnitude and phase of the oscillatingcomponent of the current is used by the computer 45 to construct imagesof the substrate surface activity.

IC-SECM imaging when operating an UME as a potentiometric tip (e.g. apH-sensitive or Cl-selective electrode or similar) produces an image ofthe concentration of the species of interest. In this case atwo-electrode potentiometric electrode set up is used (with indicatorand reference electrodes) and the potential of the indicator electrodeis measured. This can be converted to a local concentration of thespecies of interest at the location of the tip. As for amperometricimaging described above, a key advantage of this method is that thetopography of the sample and the tip-substrate separation is determinedfrom the damping of the tip oscillation. Potentiometric electrodes canalso be deployed into the IC-SECM mode for approach curve measurements.

1. A method comprising: oscillating a scanning microscopy probe tiprelative to a surface of interest; detecting damping of an amplitude ofthe oscillation of the probe tip resulting from the probe tip cominginto contact with the surface of interest; using the detected damping todetect the surface of interest; and using the probe tip to measure ormodify activity of the surface of interest simultaneously with detectingdamping.
 2. A method as claimed in claim 1, comprising detecting dampingduring an approach curve procedure.
 3. A method as claimed in claim 2,comprising terminating the approach curve process when intermittentcontact is detected.
 4. A method as claimed in claim 2, comprisingterminating the approach curve process on detecting a sustained decreasein sensor tip oscillation amplitude as compared to oscillation amplitudein a reference medium.
 5. A method as claimed in claim 2, comprisingterminating the approach curve process on detecting a decrease ofbetween 0.5% and 15% in sensor tip oscillation amplitude as compared tooscillation amplitude in a reference medium.
 6. A method as claimed inclaim 2, comprising terminating the approach curve process on detectinga decrease of about 5-10% in tip sensor oscillation amplitude ascompared to oscillation amplitude in a reference medium.
 7. A method asclaimed in claim 1, further comprising constructing an image using aseries of line scans, each line scan including a forward intermittentcontact scan and a reverse constant distance scan.
 8. A method accordingto claim 1, further comprising using a measured oscillation amplitude tocontrol the probe tip movement relative to the surface of interest.
 9. Amethod as claimed in claim 1, wherein oscillating the probe tipcomprises oscillating the probe tip with a magnitude of between 1% and2% of the radius of the active electrode.
 10. A method according toclaim 1, wherein the oscillation of the probe tip is selected from thegroup of: sinusoidal oscillation, sawtooth oscillation, and squareoscillation.
 11. (canceled)
 12. (canceled)
 13. A method according toclaim 1, in which a frequency of oscillation is selected from the group:between 5 and 100,000 Hz, between 5 and 5,000 Hz, and between 30 and 110Hz.
 14. (canceled)
 15. (canceled)
 16. A method according to claim 1, inwhich an amplitude of oscillation is selected from the group: between0.1 nm and 1 μm, between 5 nm and 500 nm, and between 15 nm and 250 nm.17. (canceled)
 18. (canceled)
 19. A method according to claim 1, inwhich the probe is selected from the group comprising: anultramicroelectrode, an ion conductance probe, a fiber optic probe, anda potentiometric indicator probe.
 20. (canceled)
 21. (canceled) 22.(canceled)
 23. A method according to claim 1, further comprising using ameasured electrochemical response of the probe to provide informationabout the surface of interest.
 24. A method according to claim 14, inwhich the electrochemical response of the probe tip is the currentgenerated at the probe tip when held at a potential to interact with aspecies of interest.
 25. A method according to claim 14, in which theelectrochemical response of the probe tip is the potential generated atthe probe tip when interacting with a species of interest.
 26. A methodaccording to claim 14, comprising using the electrochemical response ofthe probe tip to deliver chemical species to the surface of interest.27. A method as claimed in claim 1, wherein oscillating of the probe tipis normal or generally normal to the surface of interest.
 28. (canceled)29. A non-transitory computer readable medium having stored thereonmachine readable code that when executed by a processor of a scanningmicroscopy apparatus cause the apparatus to perform a method comprising:oscillating a probe tip relative to a surface of interest; detectingdamping of an amplitude of the oscillation of the probe tip resultingfrom the probe tip coming into contact with the surface of interest;using the detected damping to detect the surface of interest; and usingthe probe tip to measure or modify activity of the surface of interestsimultaneously with detecting damping.
 30. Apparatus configured: tooscillate a scanning microscopy probe tip relative to a surface ofinterest; to detect damping of an amplitude of the oscillation of theprobe tip resulting from the probe tip coming into contact with thesurface of interest; to use the detected damping to detect the surfaceof interest; and to use the probe tip to measure or modify activity ofthe surface of interest simultaneously with detecting damping. 31.Apparatus comprising: means for oscillating a scanning microscopy probetip relative to a surface of interest; means for detecting damping of anamplitude of the oscillation of the probe tip resulting from the probetip coming into contact with the surface of interest; means for usingthe detected damping to detect the surface of interest; and means forusing the probe tip to measure or modify activity of the surface ofinterest simultaneously with detecting damping.